Michel Barnier said Europe had no intention of questioning the UK's constitutional order but was seeking practical solutions to a complex problem.

Many operational details have yet to be resolved surrounding the vexed question of the UK's only land border with an EU state after Brexit and the issue is top of the agenda in Brussels, Mr Barnier has reiterated.

He said: "We need to agree rapidly by June on the scope of all-island customs and regulations, the safety and controls that we need to respect the single market."

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This summer's meeting of European leaders in Brussels would be a "stepping stone" for the final summit in October, which is the deadline for reaching an agreement on withdrawal, he added.

A joint report on the UK's withdrawal agreed in December by Prime Minister Theresa May and European Commission president Jean-Claude Juncker included both British proposals, along with a third "backstop" option which would keep Northern Ireland in the customs union.

We are seeking practical, practical and operational, solutions to a complex problem. No more, no less. We are seeking practical, practical and operational, solutions to a complex problem. No more, no less.

(Image: PA)

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But a version published by the EU in February and agreed by the EU27 last month contained only the "backstop", effectively drawing a customs border down the Irish Sea, which a furious Mrs May said "no British prime minister could ever agree".

Mr Barnier explained: "We have no intention of questioning the UK's constitutional order. That is none of our business.

"We are seeking practical, practical and operational, solutions to a complex problem. No more, no less."

He added: "The backstop is not there to change the UK's red lines. It is there because of the UK's red lines.

"The UK's decision to leave the single market and the customs union creates a risk that the hard border will return. This is why it is necessary to have a self-standing backstop solution."

He visited the Irish border town of Dundalk, Co Louth, for a conference organised by the Irish Government on Monday.

"We want to succeed with the UK, not against the UK.

"Together with the Irish government we are looking for practical solutions."

This week marks his third visit to Ireland and Irish foreign affairs minister Simon Coveney said he was a friend of the country.

After delivering the keynote speech during a meeting of the Irish government-hosted All-Island Civic Dialogue on Brexit in Dundalk, Mr Barnier crosses the frontier to Newry in Co Down for meetings with business leaders.

On Tuesday he will visit the other, north-western end of the porous 310-mile border at Londonderry.

Arlene Foster (Image: PA)

DUP leader Arlene Foster has said Mr Barnier did not understand unionist culture.

She told the BBC: "He's hearing a very strong message from the Republic of Ireland's government, he's hearing it from Sinn Fein.

"We have tried to get him to understand the unionist position for the people of Northern Ireland, but he hasn't really responded to that and I'm disappointed about that."

In response, Mr Barnier said: "My door is open."

Asked if the visit was "helpful", the Prime Minister's official spokesman said: "It's a matter for Michel Barnier where he chooses to spend his Monday mornings."